1. Field of the Invention
The present invention relates to a device and a method for forming a bent pipe of a non-annular cross-section used, for example, as intake manifolds.
2. Description of the Prior Art
In Japanese Patent Laid Open Print Number Hei.2(1990)-268927, which was published on Nov. 2, 1990, without examination, there is disclosed a pipe bending device by which a straight pipe is bent into a bent pipe with a curvature. This device includes a round bar shaped guide member, and a floating member connected to a distal end of the round bar shaped guide member in a ball-and-socket joint manner. The floating member has a circular portion whose radius is larger than that of the straight pipe. While the straight pipe slides on the round bar-shaped guide member toward its distal end, the straight pipe, under movement, is being fitted with the floating member, thereby expanding the radius of the straight pipe. The resultant pipe is applied with a force from a bending device and, thus, the straight pipe with the expanded radius is Formed into the bent pipe having the curvature.
In Japanese Patent Publication Number Sho. 51(1976)-7151, published on Mar. 5, 1976, after examination, there is disclosed a device including a round, bar-shaped guide member and an expanding member connected fixedly to a distal end of the round, bar-shaped guide member. The member has a circular portion whose radius is larger than that of a straight pipe having an annular cross-section. While the straight pipe slides on the round, bar-shaped guide member toward its distal end, the straight pipe, under movement, is being fitted with the expanding member, thereby expanding the radius of the pipe. This pipe is also bent into a curvature. This reference suggests literally that changing a cross-section of such an expanding member will enable the obtaining of a bent pipe having a non-annular cross-section from a straight pipe having an annular cross-section.
By reforming the cross-section of the floating member in the former disclosure into a prismoid structure at the suggestion of the latter disclosure, a device was found for obtaining a bent pipe which is of a non-annular cross-section from a straight pipe. However, bent pipes obtained by that device were unsatisfactory for the reason that cracks, destroyed portions, rumples and/or spring backs were found in the bent pipe.